A photo-detecting apparatus using CMOS technology is known; especially, the one of an active-pixel type system is known (see Patent Reference 1). The photo-detecting apparatus of the active-pixel system has a picture element or pixel section of an active-pixel type including a photodiode for generating the amount of an electric charge corresponding to intensity of incident light, and converts the electric charge generated at the photodiode into a voltage, namely carries out charge-to-voltage conversion by way of a source follower circuit constituting transistors, which enables photodetection with high sensitivity and low noise.
An output voltage value V is represented by the following equation: V=Q/Cf, where Cf is the capacitance value of the parasitic capacitance section that stores the electric charge generated at the photodiode, and Q is the amount of the electric charge. As is apparent from this equation, when the capacitance value Cf is decreased, sensitivity of the photodetection can be enhanced.
On the other hand, an upper limit of the output voltage value V is a level of several voltages due to the range of an applicable voltage of a power source and circuit-based restrictions. Thus, there is also an upper limit in the amount Q of the electric charge that can be accumulated in the parasitic capacitance section.
Provided that the upper limit of the amount Q of the electric charge that can be accumulated in the parasitic capacitance section (the amount of saturated electric charge) is enhanced, it is assumed to increase the capacitance value Cf, or a voltage value of the power source. However, in order to increase the capacitance value Cf of the parasitic capacitance section, it is necessary to be manufactured through fine CMOS processes, so that a smaller voltage value of the power source is necessary; after all, it is impossible to increase the amount of the saturated electric charge. Additionally, if the capacitance value Cf of the parasitic capacitance section is increased, an important advantage of high sensitivity may be lost.
Patent Reference 1: JP-A-11-274454